Motor starters using pulse sequences with subfundamental modulation

Abstract

A motor starter includes a first switch configured to couple and decouple an AC power source to a motor, a second switch configured to bypass the first switch, and a control circuit configured to operate the first switch to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage and to operate the second switch to bypass the first switch responsive to operation of the motor meeting a predetermined criterion.

That which is claimed: 
     
         1 . A method of starting a motor, the method comprising:
 operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage; and   bypassing the switch to couple the AC power source to the motor responsive to operation of the motor meeting a predetermined criterion.   
     
     
         2 . The method of  claim 1 , wherein operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage comprises providing a series of pulses of the AC voltage that occur at the fundamental frequency and have varying pulse durations. 
     
     
         3 . The method of  claim 1 , wherein operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage comprises:
 modulating the AC voltage at a first frequency; and   modulating the AC voltage at a second frequency greater than the first frequency and less than the fundamental frequency.   
     
     
         4 . The method of  claim 3 , wherein modulating the AC voltage at a second frequency greater than the first frequency and less than the fundamental frequency comprises changing from modulating at the first frequency to modulating at the second frequency responsive to meeting a predetermined criterion. 
     
     
         5 . The method of  claim 4 , further comprising sensing a speed of the motor using a speed sensor and/or back electromotive force (EMF) and wherein changing from modulating at the first frequency to modulating at the second frequency responsive to meeting a predetermined criterion comprises changing from modulating at the first frequency to modulating at the second frequency responsive to the sensed speed meeting a predetermined criterion. 
     
     
         6 . The method of  claim 3 , wherein modulating at the second frequency applies a greater RMS voltage to the motor than modulating at the first frequency. 
     
     
         7 . The method of  claim 3 , wherein the first and second frequencies are subharmonics of the fundamental frequency. 
     
     
         8 . The method of  claim 1 , wherein operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage comprises operating the switch to modulate an AC voltage applied to the motor from the AC power source at a frequency that is a subharmonic of the fundamental frequency. 
     
     
         9 . The method of  claim 1 , wherein operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage comprises operating the switch to apply a voltage of a first polarity in a first half of a modulation cycle and to apply a voltage of a second polarity in a second half of a modulation cycle. 
     
     
         10 . A motor starter comprising:
 a first switch configured to couple and decouple an AC power source to and from a motor;   a second switch configured to bypass the first switch; and   a control circuit configured to operate the first switch to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage and to operate the second switch to bypass the first switch responsive to operation of the motor meeting a predetermined criterion.   
     
     
         11 . The motor starter of  claim 10 , wherein the control circuit is configured modulate the AC voltage at the frequency that is less than the fundamental frequency of the AC voltage by providing a series of pulses of the AC voltage that occur at the fundamental frequency and have varying pulse durations. 
     
     
         12 . The motor starter of  claim 1 , wherein the control circuit is configured to modulating the AC voltage at a first frequency and then modulate the AC voltage at a second frequency greater than the first frequency and less than the fundamental frequency. 
     
     
         13 . The motor starter of  claim 12 , wherein the control circuit is configured to change from modulating at the first frequency to modulating at the second frequency responsive to meeting a predetermined criterion. 
     
     
         14 . The motor starter of  claim 13 , wherein the control circuit is configured to sense a speed of the motor using a speed sensor and/or back electromotive force (EMF) and to change from modulating at the first frequency to modulating at the second frequency responsive to the sensed speed meeting a predetermined criterion. 
     
     
         15 . The motor starter of  claim 12 , wherein the modulating at the second frequency applies a greater RMS voltage to the motor than modulating at the first frequency. 
     
     
         16 . The motor starter of  claim 12 , wherein the first and second frequencies are subharmonics of the fundamental frequency. 
     
     
         17 . The motor starter of  claim 10 , wherein the control circuit is configured to modulate the AC voltage at a frequency that is a subharmonic of the fundamental frequency. 
     
     
         18 . The motor starter of  claim 10 , wherein the control circuit is configured to apply a voltage of a first polarity during a first half of a modulation cycle and to apply a voltage of a second polarity during a second half of a modulation cycle.